There is currently extensive development for a CCD (charge coupled device) image sensor having higher resolution but smaller size. This increases the number and decreases the width of transfer electrodes constituting a CCD shift register. For the driver circuit to drive the CCD shift register, it is necessary to increase the pulse amplitude and the drive capability. Particularly, in a frame-transfer CCD image sensor, frame-transfer operation is performed at a high rate from the imaging section to the storage section. Consequently, high drive capability is needed for the driver circuit for use in a frame-transfer CCD image sensor.
FIG. 1 is a typical circuit diagram showing a configuration of a current driver circuit. A timing circuit 2 generates various clocks at suitable timings for the operation of the CCD image sensor. Output pulses from the timing circuit 2 are input to preceding-stage circuits 4 (4a, 4b). The preceding-stage circuits 4 are each an inverter circuit formed by P-channel and N-channel MOS field-effect transistors (P-ch and N-ch MOSFETs) connected between a positive voltage source VDD and a negative voltage source VLOW, to generate and output a pulse inverted in voltage magnitude relation relative to the pulse inputted to the gates of the MOSFETs from the timing circuit 2. Output pulses from the preceding-stage circuit 4 are input to an output-stage circuit 6. The output-stage circuit 6 is also an inverter circuit whose P-ch and N-ch MOSFETs are connected between VDD and VLOW. This generates a pulse inverted in voltage magnitude relation relative to the input pulse from the preceding-stage circuits 4 and supplies it to the CCD image sensor.
Specifically, when each of the preceding-stage circuits 4 is supplied with an input of VH from the timing circuit 2, its N-ch MOSFET having a source connected to VLOW turns on so that the preceding-stage circuit 4 outputs a voltage comparable with VLOW. Meanwhile, when each of the preceding-stage circuits 4 is supplied with an input of VL (<VH) from the timing circuit 2, the P-ch MOSFET having a source connected to VDD turns on so that the preceding-stage circuit 4 outputs a voltage comparable with VDD. When the voltage comparable with VDD is input to the output-stage circuit 6 from the preceding-stage circuit 4b, the N-ch MOSFET having a source connected to VLOW turns on. The N-ch MOSFET in on state sets the voltage, on the output terminal Vout connected to the CCD image sensor, to the voltage comparable with VLOW and supplies, to Vout, a current according to its gate-to-source voltage Vgs. Meanwhile, when the voltage comparable with VLOW is input to the output-stage circuit 6 from the preceding-stage circuit 4a, the P-ch MOSFET having a source connected to VDD turns on. The P-ch MOSFET in on state sets the voltage of Vout at a value comparable with VDD and supplies, to Vout, a current according to its gate-to-source voltage Vgs.
Namely, the current driver circuit has an output whose voltage amplitude ranges from VLOW to VDD while each of the MOSFETs of the output-stage circuit 6 has Vgs at (VDD−VLOW).
In current driver circuits, in order to raise the drive capability while maintaining the MOSFET Vgs of the output-stage circuit 6 at (VDD−VLOW), the transistor must be increased in size, e.g. increasing the MOSFET channel width in the output-stage circuit 6. Thus, there is encountered a problem of chip size increase in the driver circuit.